Pioneer patents in the filed of microlithography projection printing include the patents of R. M. Scott, U.S. Pat. No. 3,821,763, and A. Offner, U.S. Pat. No. 3,748,015. The Scott patent shows a restricted off-axis field optical system in which an annular slit is used to restrict the field to an annular zone centered on the optical axis, and the system is optically corrected to preferentially increase image quality in the annular zone. The Offner patent discloses a catoptric, off-axis annular field optical system for forming in accurate micro detail an image of an object at unit magnification with high resolution, including convex and concave mirrors in face-to-face relationships with their centers of curvature being nearly concentric. Another high performance optical system is disclosed in the A. Offner U.S. Pat. No. 4,293,186, which includes a system having refractive elements and having means for obtaining stigmatic imagery in the restricted off-axis field over an extended spectral range by balancing the chromatic variation in focus at the center of the restricted off-axis field due to the variation of field curvature with color by introducing axial color aberration of the opposite sense.
Meniscus elements can be used to reduce or remove the spherical aberration of principal rays parallel to the optical axis, see the publication entitled "Achievements in Optics" by A. Bouwers, Elsevier Publishing Company, Inc., 1946 (particularly pages 24, 25 and 39) and the article appearing in the Journal of the Optical Society of America, Vol. 34 No. 5, May 1944, pp. 270-284, entitled: "New Catadioptric Meniscus System" by D. D. Maksutov. Methods of correcting the axial longitudinal color aberration of said systems are also described in these references.
Other related patents and publications in this field include: Russian Pat. No. 126,911, issued Mar. 30, 1959; French Pat. No. 784,063 issued July 22, 1935; U.S. Pat. No. 3,244,073 issued Apr. 5, 1966; U.S. Pat. No. 3,821,763 issued June 28, 1974; U.S. Pat. No. 3,951,546 issued Apr. 20, 1976 and U.S. Pat. No. 4,011,011 issued Mar 8, 1977, which is a continuation of U.S. patent application Ser. No. 339,860, filed Mar. 9, 1973 (now abandoned); U.S. Pat. No. 4,288,148 issued Sept. 8, 1981; "Unit Magnification Optical System Without Seidel Aberrations", by J. Dyson, Journal of the Optical Society of America, Vol. 49, pp. 713, 1959; U.S. patent application Ser. No. 308,647 filed Oct. 5, 1981 entitled "Method and Apparatus for Optical System Adjustments", which shows means and method for correcting an image forming optical system for deviations from the theoretical design performance resulting from manufacturing errors in its optical components and errors in their assembly and/or alignment.
A particularly desirable restricted off-axis field optical system having a broad spectral range is disclosed in patent application Ser. No. 383,683 filed June 1, 1982, which describes an optical system comprising first and second substantially concentric optical subsystems, which are constructed and arranged with respect to each other so that variation in performance of one subsystem with wavelength substantially balances that of the other. That is, the subsystems are constructed and arranged with respect to each other so that the sum of the refractive powers is nearly zero and the sum of the reflective powers is also nearly zero.
The term "nearly" and the term "nearly concentric" are intended to cover installations where there are two points or two centers which are made as close to being coincident as is physically possible to achieve, and it is also intended to cover the installations where the points or centers are very close together but purposely slightly spaced one from another.